This trait provides an interface that allows suites of tests to be run. Its implementation enables a default way of writing and executing tests. Subtraits and subclasses can override Suite's methods to enable other ways of writing and executing tests. This trait's default approach allows tests to be defined as methods whose name starts with "test." This approach is easy to understand, and a good way for Scala beginners to start writing tests. More advanced Scala programmers may prefer to mix together other Suite subtraits defined in ScalaTest, or create their own, to write tests in the way they feel makes them most productive. Here's a quick overview of some of the options to help you get started:

For JUnit 3 users

If you are using JUnit 3 (version 3.8 or earlier releases) and you want to write JUnit 3 tests in Scala, look at AssertionsForJUnit, ShouldMatchersForJUnit, and JUnit3Suite.

For JUnit 4 users

If you are using JUnit 4 and you want to write JUnit 4 tests in Scala, look at JUnitSuite, and JUnitRunner. With JUnitRunner, you can use any of the traits described here and still run your tests with JUnit 4.

For TestNG users

If you are using TestNG and you want to write TestNG tests in Scala, look at TestNGSuite.

For high-level testing

If you want to write tests at a higher level than unit tests, such as integration tests, acceptance tests, or functional tests, check out FeatureSpec.

For unit testing

If you prefer a behavior-driven development (BDD) style, in which tests are combined with text that specifies the behavior being tested, look at Spec, FlatSpec, and WordSpec. Otherwise, if you just want to write tests and don't want to combine testing with specifying, look at FunSuite or read on to learn how to write tests using this base trait, Suite.

To use this trait's approach to writing tests, simply create classes that extend Suite and define test methods. Test methods have names of the form testX, where X is some unique, hopefully meaningful, string. A test method must be public and can have any result type, but the most common result type is Unit. Here's an example:

 import org.scalatest.Suite

 class MySuite extends Suite {

   def testAddition() {
     val sum = 1 + 1
     assert(sum === 2)
     assert(sum + 2 === 4)
   }

   def testSubtraction() {
     val diff = 4 - 1
     assert(diff === 3)
     assert(diff - 2 === 1)
   }
 }
 

You can run a Suite by invoking on it one of four overloaded execute methods. These methods, which print test results to the standard output, are intended to serve as a convenient way to run tests from within the Scala interpreter. For example, to run MySuite from within the Scala interpreter, you could write:

 scala> (new MySuite).execute()
 

And you would see:

 Test Starting - MySuite: testAddition
 Test Succeeded - MySuite: testAddition
 Test Starting - MySuite: testSubtraction
 Test Succeeded - MySuite: testSubtraction
 

Or, to run just the testAddition method, you could write:

 scala> (new MySuite).execute("testAddition")
 

And you would see:

 Test Starting - MySuite: testAddition
 Test Succeeded - MySuite: testAddition
 

Two other execute methods that are intended to be run from the interpreter accept a "config" map of key-value pairs (see Config map, below). Each of these execute methods invokes a run method takes seven parameters. This run method, which actually executes the suite, will usually be invoked by a test runner, such as org.scalatest.tools.Runner or an IDE. See the documentation for Runner for more detail.

Assertions and ===

Inside test methods in a Suite, you can write assertions by invoking assert and passing in a Boolean expression, such as:

 val left = 2
 val right = 1
 assert(left == right)
 

If the passed expression is true, assert will return normally. If false, assert will complete abruptly with a TestFailedException. This exception is usually not caught by the test method, which means the test method itself will complete abruptly by throwing the TestFailedException. Any test method that completes abruptly with a TestFailedException or any Exception is considered a failed test. A test method that returns normally is considered a successful test.

If you pass a Boolean expression to assert, a failed assertion will be reported, but without reporting the left and right values. You can alternatively encode these values in a String passed as a second argument to assert, as in:

 val left = 2
 val right = 1
 assert(left == right, left + " did not equal " + right)
 

Using this form of assert, the failure report will include the left and right values, thereby helping you debug the problem. However, ScalaTest provides the === operator to make this easier. (The === operator is defined in trait Assertions which trait Suite extends.) You use it like this:

 val left = 2
 val right = 1
 assert(left === right)
 

Because you use === here instead of ==, the failure report will include the left and right values. For example, the detail message in the thrown TestFailedException from the assert shown previously will include, "2 did not equal 1". From this message you will know that the operand on the left had the value 2, and the operand on the right had the value 1.

If you're familiar with JUnit, you would use === in a ScalaTest Suite where you'd use assertEquals in a JUnit TestCase. The === operator is made possible by an implicit conversion from Any to Equalizer. If you're curious to understand the mechanics, see the documentation for Equalizer and the convertToEqualizer method.

Expected results

 val a = 5
 val b = 2
 expect(2) {
   a - b
 }
 

In this case, the expected value is 2, and the code being tested is a - b. This expectation will fail, and the detail message in the TestFailedException will read, "Expected 2, but got 3."

Intercepted exceptions

Sometimes you need to test whether a method throws an expected exception under certain circumstances, such as when invalid arguments are passed to the method. You can do this in the JUnit style, like this:

 val s = "hi"
 try {
   s.charAt(-1)
   fail()
 }
 catch {
   case _: IndexOutOfBoundsException => // Expected, so continue
 }
 

If charAt throws IndexOutOfBoundsException as expected, control will transfer to the catch case, which does nothing. If, however, charAt fails to throw an exception, the next statement, fail(), will be executed. The fail method always completes abruptly with a TestFailedException, thereby signaling a failed test.

To make this common use case easier to express and read, ScalaTest provides an intercept method. You use it like this:

 val s = "hi"
 intercept[IndexOutOfBoundsException] {
   s.charAt(-1)
 }
 

This code behaves much like the previous example. If charAt throws an instance of IndexOutOfBoundsException, intercept will return that exception. But if charAt completes normally, or throws a different exception, intercept will complete abruptly with a TestFailedException. The intercept method returns the caught exception so that you can inspect it further if you wish, for example, to ensure that data contained inside the exception has the expected values. Here's an example:

 val s = "hi"
 val caught =
   intercept[IndexOutOfBoundsException] {
     s.charAt(-1)
   }
 assert(caught.getMessage === "String index out of range: -1")
 

Using other assertions

ScalaTest also supports another style of assertions via its matchers DSL. By mixing in trait ShouldMatchers, you can write suites that look like:

 import org.scalatest.Suite
 import org.scalatest.matchers.ShouldMatchers

 class MySuite extends Suite with ShouldMatchers {

   def testAddition() {
     val sum = 1 + 1
     sum should equal (2)
     sum + 2 should equal (4)
   }

   def testSubtraction() {
     val diff = 4 - 1
     diff should equal (3)
     diff - 2 should equal (1)
   }
 }
 

If you prefer the word "must" to the word "should," you can alternatively mix in trait MustMatchers.

If you are comfortable with assertion mechanisms from other test frameworks, chances are you can use them with ScalaTest. Any assertion mechanism that indicates a failure with an exception can be used as is with ScalaTest. For example, to use the assertEquals methods provided by JUnit or TestNG, simply import them and use them. (You will of course need to include the relevant JAR file for the framework whose assertions you want to use on either the classpath or runpath when you run your tests.) Here's an example in which JUnit's assertions are imported, then used within a ScalaTest suite:

 import org.scalatest.Suite
 import org.junit.Assert._

 class MySuite extends Suite {

   def testAddition() {
     val sum = 1 + 1
     assertEquals(2, sum)
     assertEquals(4, sum + 2)
   }

   def testSubtraction() {
     val diff = 4 - 1
     assertEquals(3, diff)
     assertEquals(1, diff - 2)
   }
 }
 

Nested suites

A Suite can refer to a collection of other Suites, which are called nested Suites. Those nested Suites can in turn have their own nested Suites, and so on. Large test suites can be organized, therefore, as a tree of nested Suites. This trait's run method, in addition to invoking its test methods, invokes run on each of its nested Suites.

A List of a Suite's nested Suites can be obtained by invoking its nestedSuites method. If you wish to create a Suite that serves as a container for nested Suites, whether or not it has test methods of its own, simply override nestedSuites to return a List of the nested Suites. Because this is a common use case, ScalaTest provides a convenience SuperSuite class, which takes a List of nested Suites as a constructor parameter. Here's an example:

 import org.scalatest.Suite

 class ASuite extends Suite
 class BSuite extends Suite
 class CSuite extends Suite

 class AlphabetSuite extends SuperSuite(
   List(
     new ASuite,
     new BSuite,
     new CSuite
   )
 )
 

If you now run AlphabetSuite, for example from the interpreter:

 scala> (new AlphabetSuite).run()
 

You will see reports printed to the standard output that indicate nested suites—ASuite, BSuite, and CSuite—were run.

Note that Runner can discover Suites automatically, so you need not necessarily specify SuperSuites explicitly. See the documentation for Runner for more information.

Shared fixtures

A test fixture is objects or other artifacts (such as files, sockets, database connections, etc.) used by tests to do their work. If a fixture is used by only one test method, then the definitions of the fixture objects can be local to the method, such as the objects assigned to sum and diff in the previous MySuite examples. If multiple methods need to share an immutable fixture, one approach is to assign them to instance variables. Here's a (very contrived) example, in which the object assigned to shared is used by multiple test methods:

 import org.scalatest.Suite

 class MySuite extends Suite {

   // Sharing immutable fixture objects via instance variables
   val shared = 5

   def testAddition() {
     val sum = 2 + 3
     assert(sum === shared)
   }

   def testSubtraction() {
     val diff = 7 - 2
     assert(diff === shared)
   }
 }
 

In some cases, however, shared mutable fixture objects may be changed by test methods such that they need to be recreated or reinitialized before each test. Shared resources such as files or database connections may also need to be created and initialized before, and cleaned up after, each test. JUnit 3 offers methods setUp and tearDown for this purpose. In ScalaTest, you can use the BeforeAndAfterEach trait, which will be described later, to implement an approach similar to JUnit's setUp and tearDown, however, this approach usually involves reassigning vars between tests. Before going that route, you may wish to consider some approaches that avoid vars. One approach is to write one or more create-fixture methods that return a new instance of a needed object (or a tuple or case class holding new instances of multiple objects) each time it is called. You can then call a create-fixture method at the beginning of each test method that needs the fixture, storing the fixture object or objects in local variables. Here's an example:

 import org.scalatest.Suite
 import scala.collection.mutable.ListBuffer

 class MySuite extends Suite {

   // create objects needed by tests and return as a tuple
   def createFixture = (
     new StringBuilder("ScalaTest is "),
     new ListBuffer[String]
   )

   def testEasy() {
     val (builder, lbuf) = createFixture
     builder.append("easy!")
     assert(builder.toString === "ScalaTest is easy!")
     assert(lbuf.isEmpty)
     lbuf += "sweet"
   }

   def testFun() {
     val (builder, lbuf) = createFixture
     builder.append("fun!")
     assert(builder.toString === "ScalaTest is fun!")
     assert(lbuf.isEmpty)
   }
 }
 

If different tests in the same Suite require different fixtures, you can create multiple create-fixture methods and call the method (or methods) needed by each test at the begining of the test. If every test method requires the same set of mutable fixture objects, one other approach you can take is make them simply vals and mix in trait OneInstancePerTest. If you mix in OneInstancePerTest, each test will be run in its own instance of the Suite, similar to the way JUnit tests are executed.

Although the create-fixture and OneInstancePerTest approaches take care of setting up a fixture before each test, they don't address the problem of cleaning up a fixture after the test completes. In this situation, one option is to mix in the BeforeAndAfterEach trait. BeforeAndAfterEach's beforeEach method will be run before, and its afterEach method after, each test (like JUnit's setUp and tearDown methods, respectively). For example, you could create a temporary file before each test, and delete it afterwords, like this:

 import org.scalatest.Suite
 import org.scalatest.BeforeAndAfterEach
 import java.io.FileReader
 import java.io.FileWriter
 import java.io.File

 class MySuite extends Suite with BeforeAndAfterEach {

   private val FileName = "TempFile.txt"
   private var reader: FileReader = _

   // Set up the temp file needed by the test
   override def beforeEach() {
     val writer = new FileWriter(FileName)
     try {
       writer.write("Hello, test!")
     }
     finally {
       writer.close()
     }

     // Create the reader needed by the test
     reader = new FileReader(FileName)
   }

   // Close and delete the temp file
   override def afterEach() {
     reader.close()
     val file = new File(FileName)
     file.delete()
   }

   def testReadingFromTheTempFile() {
     var builder = new StringBuilder
     var c = reader.read()
     while (c != -1) {
       builder.append(c.toChar)
       c = reader.read()
     }
     assert(builder.toString === "Hello, test!")
   }

   def testFirstCharOfTheTempFile() {
     assert(reader.read() === 'H')
   }
 
   def testWithoutAFixture() {
     assert(1 + 1 === 2)
   }
 }
 

In this example, the instance variable reader is a var, so it can be reinitialized between tests by the beforeEach method.

Although the BeforeAndAfterEach approach should be familiar to the users of most test other frameworks, ScalaTest provides another alternative that also allows you to perform cleanup after each test: overriding withFixture(NoArgTest). To execute each test, Suite's implementation of the runTest method wraps an invocation of the appropriate test method in a no-arg function. runTest passes that test function to the withFixture(NoArgTest) method, which is responsible for actually running the test by invoking the function. Suite's implementation of withFixture(NoArgTest) simply invokes the function, like this:

 // Default implementation
 protected def withFixture(test: NoArgTest) {
   test()
 }
 

The withFixture(NoArgTest) method exists so that you can override it and set a fixture up before, and clean it up after, each test. Thus, the previous temp file example could also be implemented without mixing in BeforeAndAfterEach, like this:

 import org.scalatest.Suite
 import java.io.FileReader
 import java.io.FileWriter
 import java.io.File

 class MySuite extends Suite {

   private var reader: FileReader = _

   override def withFixture(test: NoArgTest) {

     val FileName = "TempFile.txt"

     // Set up the temp file needed by the test
     val writer = new FileWriter(FileName)
     try {
       writer.write("Hello, test!")
     }
     finally {
       writer.close()
     }

     // Create the reader needed by the test
     reader = new FileReader(FileName)

     try {
       test() // Invoke the test function
     }
     finally {
       // Close and delete the temp file
       reader.close()
       val file = new File(FileName)
       file.delete()
     }
   }

   def testReadingFromTheTempFile() {
     var builder = new StringBuilder
     var c = reader.read()
     while (c != -1) {
       builder.append(c.toChar)
       c = reader.read()
     }
     assert(builder.toString === "Hello, test!")
   }

   def testFirstCharOfTheTempFile() {
     assert(reader.read() === 'H')
   }
 
   def testWithoutAFixture() {
     assert(1 + 1 === 2)
   }
 }
 

If you prefer to keep your test classes immutable, one final variation is to use the FixtureSuite trait from the org.scalatest.fixture package. Tests in an org.scalatest.fixture.FixtureSuite can have a fixture object passed in as a parameter. You must indicate the type of the fixture object by defining the Fixture type member and define a withFixture method that takes a one-arg test function. (A FixtureSuite has two overloaded withFixture methods, therefore, one that takes a OneArgTest and the other, inherited from Suite, that takes a NoArgTest.) Inside the withFixture(OneArgTest) method, you create the fixture, pass it into the test function, then perform any necessary cleanup after the test function returns. Instead of invoking each test directly, a FixtureSuite will pass a function that invokes the code of a test to withFixture(OneArgTest). Your withFixture(OneArgTest) method, therefore, is responsible for actually running the code of the test by invoking the test function. For example, you could pass the temp file reader fixture to each test that needs it by overriding the withFixture(OneArgTest) method of a FixtureSuite, like this:

 import org.scalatest.fixture.FixtureSuite
 import java.io.FileReader
 import java.io.FileWriter
 import java.io.File
 
 class MySuite extends FixtureSuite {

   // No vars needed in this one

   type FixtureParam = FileReader

   def withFixture(test: OneArgTest) {

     val FileName = "TempFile.txt"

     // Set up the temp file needed by the test
     val writer = new FileWriter(FileName)
     try {
       writer.write("Hello, test!")
     }
     finally {
       writer.close()
     }

     // Create the reader needed by the test
     val reader = new FileReader(FileName)
  
     try {
       // Run the test, passing in the temp file reader
       test(reader)
     }
     finally {
       // Close and delete the temp file
       reader.close()
       val file = new File(FileName)
       file.delete()
     }
   }
 
   def testReadingFromTheTempFile(reader: FileReader) {
     var builder = new StringBuilder
     var c = reader.read()
     while (c != -1) {
       builder.append(c.toChar)
       c = reader.read()
     }
     assert(builder.toString === "Hello, test!")
   }
 
   def testFirstCharOfTheTempFile(reader: FileReader) {
     assert(reader.read() === 'H')
   }
 
   def testWithoutAFixture() {
     assert(1 + 1 === 2)
   }
 }
 

It is worth noting that the only difference in the test code between the mutable BeforeAndAfterEach approach shown previously and the immutable FixtureSuite approach shown here is that two of the FixtureSuite's test methods take a FileReader as a parameter. Otherwise the test code is identical. One benefit of the explicit parameter is that, as demonstrated by the testWithoutAFixture method, a FixtureSuite test method need not take the fixture. (Tests that don't take a fixture as a parameter are passed to the withFixture that takes a NoArgTest, shown previously.) So you can have some tests that take a fixture, and others that don't. In this case, the FixtureSuite provides documentation indicating which test methods use the fixture and which don't, whereas the BeforeAndAfterEach approach does not.

If you want to execute code before and after all tests (and nested suites) in a suite, such as you could do with @BeforeClass and @AfterClass annotations in JUnit 4, you can use the beforeAll and afterAll methods of BeforeAndAfterAll. See the documentation for BeforeAndAfterAll for an example.

The config map

In some cases you may need to pass information to a suite of tests. For example, perhaps a suite of tests needs to grab information from a file, and you want to be able to specify a different filename during different runs. You can accomplish this in ScalaTest by passing the filename in the config map of key-value pairs, which is passed to run as a Map[String, Any]. The values in the config map are called "config objects," because they can be used to configure suites, reporters, and tests.

You can specify a string config object is via the ScalaTest Runner, either via the command line or ScalaTest's ant task. (See the documentation for Runner for information on how to specify config objects on the command line.) The config map is passed to run, runNestedSuites, runTests, and runTest, so one way to access it in your suite is to override one of those methods. If you need to use the config map inside your tests, you can use one of the traits in the org.scalatest.fixture package. (See the documentation for FixtureSuite for instructions on how to access the config map in tests.)

Tagging tests

A Suite's tests may be classified into groups by tagging them with string names. When executing a Suite, groups of tests can optionally be included and/or excluded. In this trait's implementation, tags are indicated by annotations attached to the test method. To create a new tag type to use in Suites, simply define a new Java annotation that itself is annotated with the org.scalatest.TagAnnotation annotation. (Currently, for annotations to be visible in Scala programs via Java reflection, the annotations themselves must be written in Java.) For example, to create a tag named SlowAsMolasses, to use to mark slow tests, you would write in Java:

 import java.lang.annotation.*; 
 import org.scalatest.TagAnnotation
 
 @TagAnnotation
 @Retention(RetentionPolicy.RUNTIME)
 @Target({ElementType.METHOD, ElementType.TYPE})
 public @interface SlowAsMolasses {}
 

Given this new annotation, you could place a Suite test method into the SlowAsMolasses group (i.e., tag it as being SlowAsMolasses) like this:

 @SlowAsMolasses
 def testSleeping() = sleep(1000000)
 

The primary run method takes a Filter, whose constructor takes an optional Set[String]s called tagsToInclude and a Set[String] called tagsToExclude. If tagsToInclude is None, all tests will be run except those those belonging to tags listed in the tagsToExclude Set. If tagsToInclude is defined, only tests belonging to tags mentioned in the tagsToInclude set, and not mentioned in tagsToExclude, will be run.

Note, the TagAnnotation annotation was introduced in ScalaTest 1.0, when "groups" were renamed to "tags." In 1.0 and 1.1, the TagAnnotation will continue to not be required by an annotation on a Suite method. Any annotation on a Suite method will be considered a tag until 1.2, to give users time to add TagAnnotations on any tag annotations they made prior to the 1.0 release. From 1.2 onward, only annotations themselves annotated by TagAnnotation will be considered tag annotations.

Ignored tests

Another common use case is that tests must be “temporarily” disabled, with the good intention of resurrecting the test at a later time. ScalaTest provides an Ignore annotation for this purpose. You use it like this:

 import org.scalatest.Suite
 import org.scalatest.Ignore

 class MySuite extends Suite {

   def testAddition() {
     val sum = 1 + 1
     assert(sum === 2)
     assert(sum + 2 === 4)
   }

   @Ignore
   def testSubtraction() {
     val diff = 4 - 1
     assert(diff === 3)
     assert(diff - 2 === 1)
   }
 }
 

If you run this version of MySuite with:

 scala> (new MySuite).run()
 

It will run only testAddition and report that testSubtraction was ignored. You'll see:

 Test Starting - MySuite: testAddition
 Test Succeeded - MySuite: testAddition
 Test Ignored - MySuite: testSubtraction
 

Ignore is implemented as a tag. The Filter class effectively adds org.scalatest.Ignore to the tagsToExclude Set if it not already in the tagsToExclude set passed to its primary constructor. The only difference between org.scalatest.Ignore and the tags you may define and exclude is that ScalaTest reports ignored tests to the Reporter. The reason ScalaTest reports ignored tests is as a feeble attempt to encourage ignored tests to be eventually fixed and added back into the active suite of tests.

Pending tests

A pending test is one that has been given a name but is not yet implemented. The purpose of pending tests is to facilitate a style of testing in which documentation of behavior is sketched out before tests are written to verify that behavior (and often, the before the behavior of the system being tested is itself implemented). Such sketches form a kind of specification of what tests and functionality to implement later.

To support this style of testing, a test can be given a name that specifies one bit of behavior required by the system being tested. The test can also include some code that sends more information about the behavior to the reporter when the tests run. At the end of the test, it can call method pending, which will cause it to complete abruptly with TestPendingException. Because tests in ScalaTest can be designated as pending with TestPendingException, both the test name and any information sent to the reporter when running the test can appear in the report of a test run. (In other words, the code of a pending test is executed just like any other test.) However, because the test completes abruptly with TestPendingException, the test will be reported as pending, to indicate the actual test, and possibly the functionality it is intended to test, has not yet been implemented.

Although pending tests may be used more often in specification-style suites, such as org.scalatest.Spec, you can also use it in Suite, like this:

 import org.scalatest.Suite

 class MySuite extends Suite {

   def testAddition() {
     val sum = 1 + 1
     assert(sum === 2)
     assert(sum + 2 === 4)
   }

   def testSubtraction() { pending }
 }
 

If you run this version of MySuite with:

 scala> (new MySuite).run()
 

It will run both tests but report that testSubtraction is pending. You'll see:

 Test Starting - MySuite: testAddition
 Test Succeeded - MySuite: testAddition
 Test Starting - MySuite: testSubtraction
 Test Pending - MySuite: testSubtraction
 

Informers

One of the parameters to the primary run method is an Reporter, which will collect and report information about the running suite of tests. Information about suites and tests that were run, whether tests succeeded or failed, and tests that were ignored will be passed to the Reporter as the suite runs. Most often the reporting done by default by Suite's methods will be sufficient, but occasionally you may wish to provide custom information to the Reporter from a test method. For this purpose, you can optionally include an Informer parameter in a test method, and then pass the extra information to the Informer via its apply method. The Informer will then pass the information to the Reporter by sending an InfoProvided event. Here's an example:

 import org.scalatest._
 
 class MySuite extends Suite {
   def testAddition(info: Informer) {
     assert(1 + 1 === 2)
     info("Addition seems to work")
   }
 }
 

 scala> (new MySuite).run()
 Test Starting - MySuite: testAddition(Reporter)
 Info Provided - MySuite: testAddition(Reporter)
   Addition seems to work
 Test Succeeded - MySuite: testAddition(Reporter)
 

Executing suites in parallel

The primary run method takes as its last parameter an optional Distributor. If a Distributor is passed in, this trait's implementation of run puts its nested Suites into the distributor rather than executing them directly. The caller of run is responsible for ensuring that some entity runs the Suites placed into the distributor. The -c command line parameter to Runner, for example, will cause Suites put into the Distributor to be run in parallel via a pool of threads.

Treatement of java.lang.Errors

The Javadoc documentation for java.lang.Error states:

An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.

Because Errors are used to denote serious errors, trait Suite and its subtypes in the ScalaTest API do not always treat a test that completes abruptly with an Error as a test failure, but sometimes as an indication that serious problems have arisen that should cause the run to abort. For example, if a test completes abruptly with an OutOfMemoryError, it will not be reported as a test failure, but will instead cause the run to abort. Because not everyone uses Errors only to represent serious problems, however, ScalaTest only behaves this way for the following exception types (and their subclasses):

• java.lang.annotation.AnnotationFormatError
• java.awt.AWTError
• java.nio.charset.CoderMalfunctionError
• javax.xml.parsers.FactoryConfigurationError
• java.lang.LinkageError
• java.lang.ThreadDeath
• javax.xml.transform.TransformerFactoryConfigurationError
• java.lang.VirtualMachineError

The previous list includes all Errors that exist as part of Java 1.5 API, excluding java.lang.AssertionError. ScalaTest does treat a thrown AssertionError as an indication of a test failure. In addition, any other Error that is not an instance of a type mentioned in the previous list will be caught by the Suite traits in the ScalaTest API and reported as the cause of a test failure.

Although trait Suite and all its subtypes in the ScalaTest API consistently behave this way with regard to Errors, this behavior is not required by the contract of Suite. Subclasses and subtraits that you define, for example, may treat all Errors as test failures, or indicate errors in some other way that has nothing to do with exceptions.

Extensibility

Trait Suite provides default implementations of its methods that should be sufficient for most applications, but many methods can be overridden when desired. Here's a summary of the methods that are intended to be overridden:

• run - override this method to define custom ways to run suites of tests.
• runNestedSuites - override this method to define custom ways to run nested suites.
• runTests - override this method to define custom ways to run a suite's tests.
• runTest - override this method to define custom ways to run a single named test.
• testNames - override this method to specify the Suite's test names in a custom way.
• tags - override this method to specify the Suite's test tags in a custom way.
• nestedSuites - override this method to specify the Suite's nested Suites in a custom way.
• suiteName - override this method to specify the Suite's name in a custom way.
• expectedTestCount - override this method to count this Suite's expected tests in a custom way.

For example, this trait's implementation of testNames performs reflection to discover methods starting with test, and places these in a Set whose iterator returns the names in alphabetical order. If you wish to run tests in a different order in a particular Suite, perhaps because a test named testAlpha can only succeed after a test named testBeta has run, you can override testNames so that it returns a Set whose iterator returns testBeta before testAlpha. (This trait's implementation of run will invoke tests in the order they come out of the testNames Set iterator.)

Alternatively, you may not like starting your test methods with test, and prefer using @Test annotations in the style of Java's JUnit 4 or TestNG. If so, you can override testNames to discover tests using either of these two APIs @Test annotations, or one of your own invention. (This is in fact how org.scalatest.junit.JUnitSuite and org.scalatest.testng.TestNGSuite work.)

Moreover, test in ScalaTest does not necessarily mean test method. A test can be anything that can be given a name, that starts and either succeeds or fails, and can be ignored. In org.scalatest.FunSuite, for example, tests are represented as function values. This approach might look foreign to JUnit users, but may feel more natural to programmers with a functional programming background. To facilitate this style of writing tests, FunSuite overrides testNames, runTest, and run such that you can define tests as function values.

You can also model existing JUnit 3, JUnit 4, or TestNG tests as suites of tests, thereby incorporating tests written in Java into a ScalaTest suite. The "wrapper" classes in packages org.scalatest.junit and org.scalatest.testng exist to make this easy. No matter what legacy tests you may have, it is likely you can create or use an existing Suite subclass that allows you to model those tests as ScalaTest suites and tests and incorporate them into a ScalaTest suite. You can then write new tests in Scala and continue supporting older tests in Java.